Air batteries show promising capabilities as high energy density electrochemical power sources. In its most general form, the operation of a metal/air electrochemical cell is based on the reduction of oxygen, e.g., atmospheric oxygen, which takes place at the cathode, and the oxidation of metallic anode. The aqueous electrolyte present in the cell is a highly alkaline solution, e.g., highly concentrated potassium hydroxide solution. A structure of a metal/air battery is schematically depicted in FIG. 1, in which the air cathode, the consumable metallic anode and the electrolyte are shown.
A commonly used air cathode consists of (i) an electronically conductive screen, an expanded foil or a metallic foam which serves as a current collector, (ii) active electrode particles provided within the current collector (including a catalyst for promoting the reduction of oxygen) and (iii) hydrophobic porous film (PTFE, Teflon®) supported on one face of said screen or foil. The two opposing faces of the air cathode are exposed to the atmosphere and the alkaline electrolyte, respectively. The air cathode is permeable to air, while its external face is hydrophobic and impermeable to the aqueous electrolyte.
The anode immersed in the electrolyte is made of metals such as aluminum, zinc, magnesium, iron and alloys thereof. When aluminum anode is used, then the cell is a primary cell. In the case of zinc anode, both primary and secondary cells are known.
For example, the discharge reaction for air/aluminum cell is as follows:4Al+3O2+6H2O→4Al(OH)3 
Recharging of the cell is effected by replacing the spent aluminum anode after it has been substantially consumed in the cell reaction with a fresh anode.
In its most general form, the anode applied in metal/air batteries can be defined as a spatial body bounded by a surface consisting of two opposite parallel sides (hereinafter the bases“) and lateral sides lying in planes which are typically perpendicular to the bases (hereinafter the lateral sides”). Usually, the anode has a polyhedral surface; the anode is plate-shaped, with the dimensions of the bases being, for example, from 5.0-5.0 cm*cm to 30-30 cm*cm, and a thickness ranging from 0.3 to 4.0 cm.
In use, the anode is placed in the electrochemical cell such that the bases of the anode are parallel to the active faces of the air cathodes. The bases of the anode constitute the electrochemically active areas on which the discharge and charge reactions take place. However, in addition to the desired electrochemical reactions, the anode may react directly (i.e., chemically) with the alkaline electrolyte present in the cell. The undesired chemical oxidation of the anode is chiefly due to two factors.
First, while an air battery is being charged or discharged, the lateral sides of the anode are prone to direct chemical reaction with the caustic electrolyte, leading to a useless, parasitic reaction that lowers the overall cell efficiency. The hydrogen gas which evolves as a result of the reaction may further decrease the voltage of the cell as the electrolyte conductivity decreases.
Second, when the operation of the battery is halted by the user (e.g., when the electrical vehicle in which the battery is mounted is not in motion), it is usually recommended to drain out the electrolyte of the cell compartment. It has been observed that the electrolyte cannot be completely removed from the cell compartment, and some caustic electrolyte residues are trapped in the gap between the anode plate and the casing of the cell, i.e., on the lateral sides of the anode plate. These small “electrolyte islands” which remain on the surface of the lateral sides of the anode continue to react with the anode material. The reaction is expected to proceed until the hydroxyl ions in the “electrolyte islands” are entirely consumed.
U.S. Pat. No. 4,414,293 describes parasitic corrosion-resistant anode for use in metal/air cells where the edges of the anode, i.e., the non-electrochemically active faces of the anode, are coated with a thin film of a metal which is inert in the alkali environment of the cell. The thin metal protective film, e.g., nickel, silver or gold, is deposited on the edges of the anode by electrolysis techniques.
U.S. Pat. No. 4,564,570 describes an edge seal for consumable metal anodes, especially lithium. The seal consists of a brittle polymer coating. It is reported in U.S. Pat. No. 4,564,570 that severe edge corrosion was observed when lithium anode coated on its peripheral edges with a rubbery-like plating material was exposed to an aqueous electrolyte.